1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a backlight unit for an LCD device.
2. Discussion of the Related Art
Among flat panel display devices currently in use, liquid crystal display (LCD) devices are widely applied to notebook computers, display monitors, televisions, aircraft, spacecraft, and other electronic applications. An LCD device is generally constructed of an LCD panel, a driving circuit unit, and a backlight unit.
The LCD panel includes a thin film transistor (TFT) array substrate, a color filter substrate attached to the TFT array substrate having a certain gap, and a liquid crystal layer formed therebetween. A polarizer is typically attached to an outer portion of each of the two substrates. The driving circuit unit generally includes various kinds of electric circuits mounted on a printed circuit board (PCB). The backlight unit generally includes a light emitting lamp, various kinds of optical sheets, and a supporting mold.
The LCD panel displays an image by controlling an amount of light passing therethrough. The driving circuit unit applies various kinds of signals transmitted from the electronic system to the LCD panel to thereby control the signals. The backlight unit serves as a light emitting unit for evenly irradiating light to the LCD panel.
In general, backlight units are a source of inefficiency in LCD devices because the backlight contributes to increase in thickness, weight, and power consumption. However, because the LCD panel cannot be used without a light source, research is continuing to develop backlight units that uniformly irradiate light onto a display surface while reducing their inefficiencies.
In particular, a backlight unit serving as a light source for the LCD device has to emit light having high illumination intensity while drawing minimum amounts of power. Moreover, the backlight unit needs to be able to convert linear fluorescent light into planar light so that light is irradiated evenly on every surface of the LCD device with the same brightness.
Backlight units are generally categorized as a direct-type, a side-type, or an edge-type according to the position of a light emitting lamp. In the direct-type backlight, light is irradiated from a rear surface of the LCD panel towards a front surface of the LCD panel. In the side-type backlight, light is irradiated from a side surface of a light guide plate to be transmitted to the front surface of the LCD device via the light guide plate. In the edge-type backlight, light is irradiated from one side surface of an inclined light guide plate to be transmitted to the front surface of the LCD device via the inclined light guide plate.
The side-type backlight includes a light emitting lamp for emitting light, a lamp cover for covering the light emitting lamp, and a reflection sheet for reflecting light transmitted to a rear surface of an LCD panel into a light guide plate. The light guide plate has small glass beads formed as dots on a lower surface to scatter the light from the light emitting lamp. A diffusion sheet diffuses the light illuminated from the light guide plate and prevents the dot patterns printed on the light guide plate from shining the scattered light directly to the viewer's eyes. Because intensity of the light is decreased when the light passes through the diffusion sheet, a prism sheet is used to focus the light that has passed through the diffusion sheet to increase the brightness of the light. A protection sheet is formed over the LCD device to protect the LCD device from external impact or foreign materials, and a mold frame supports the components that make up the LCD device.
Generally, the side-type backlight has a low brightness since the light emitting lamp is installed at an outer surface of the light guide plate and light passes through the light guide plate. In order to illuminate the entire surface of the liquid crystal panel, the light guide plate needs to have a high optical design and a processing technique for a uniform brightness. Accordingly, the side-type backlight is mainly used for thin LCD devices, such as a notebook computer.
The conventional direct-type backlight is typically used for LCD devices having a large screen and requiring high brightness rather than a thin profile. For these types of applications, there is no need for a light guide plate and therefore typically uses a cold cathode fluorescent lamp (CCFL) as a light emitting lamp. The light emitting lamp is typically formed as a plurality of straight pipes, a U-shaped pipe, or a W-shaped pipe.
The direct-type backlight includes a liquid crystal panel, a frame for mounting the liquid crystal panel, and a backlight housing installed in the frame. The backlight housing is provided with a light emitting lamp and a light shielding plate, a transparent film, a diffusion plate, and other optical sheets are provided over the light emitting lamp. A reflection plate typically formed of an aluminum based material is provided at a lower portion of the backlight housing.
The light shielding plate is arranged on the light emitting lamp in order to control the brightness of light emitted from the light emitting lamp. The light shielding plate usually has dots printed on a poly-ethylene film, for example, according to the shape of the light emitting lamp. The transparent film is arranged on the light shielding plate, and the diffusion plate is arranged on the transparent film. The transparent film forms an optical space between the diffusion plate and the light emitting lamp.
Light that has passed through the light shielding plate is made to be incident on the diffusion plate with a wide angle, thereby enhancing a diffusion characteristic of the light. The diffusion plate is made of an optical material coated on both surfaces of a film formed of a transparent resin, for example. The backlight is inserted into the frame where the LCD panel, the printed circuit board, and other related components are mounted.
One way of supplying power to a backlight unit is to have a plurality of light emitting lamps connected to one another arranged on a flat plate. However, when an alternating current is applied to the flat plate, only a few lamps emit light. Therefore, each of the light emitting lamps has to be driven by an individual inverter. In other words, power needs to be supplied to each of the light emitting lamps individually.
In previous conventional direct-type backlight units, a cold cathode fluorescent lamp (CCFL) having a high optical efficiency was used as a light emitting lamp. However, because a plurality of CCFLs cannot readily be connected to one another in parallel, external electrode fluorescent lamps (EEFL) are currently being used as the light emitting lamp. The EEFLs can be arranged on a plane in parallel, and thus can be connected to one power source for driving.
A structure of the conventional direct type backlight unit will be explained with reference to FIGS. 1 and 2. FIG. 1 is a cross-sectional view showing an LCD device having a direct-type backlight unit in accordance with a related art. FIG. 2 is a cross-sectional, close up view showing a lower frame of the LCD device having the direct-type backlight unit in accordance with the related art.
As shown in FIG. 1, the direct-type backlight unit of the related art comprises a lower frame 11 for mounting components of a backlight, a reflection plate 13 installed at a surface of the lower frame 11, a plurality of light emitting lamps 15 arranged above the reflection plate 13 with a certain gap, a diffusion plate 17 arranged above the light emitting lamps 15 with a certain gap, a plurality of optical sheets 19 formed on the diffusion plate 17, a guide panel 21 formed at each edge of the lower frame 11, and a liquid crystal display (LCD) panel 31 mounted at a panel mounting portion of the guide panel 21. Polarizers 33 and 35 are provided at a lower surface and an upper surface of the LCD panel 31, respectively. An upper frame 41 covering edges of the LCD panel 31 except a screen display portion is coupled to the guide panel 21 and the lower frame 11.
As shown in FIG. 2, the lower frame 11 has of a flat unit supporting portion 11a and a tapered panel supporting portion 11b. The lower frame 11 is formed of a metal material having an excellent thermal conductivity. An upper surface of the edge 11b of the lower frame 11 is formed to be flat so that the guide panel 21 can be mounted thereon. In turn, the LCD panel 31 is mounted on the guide panel 21.
A path of light transmitted to the LCD panel 31 in the direct-type backlight of the related art will be explained. Most of the light emitted from the light emitting lamps 15 is made to be directly incident on the diffusion plate 17. Some of the light is partially reflected by the reflection plate 13, thereby made to be incident on the diffusion plate 17. The reflection plate 13 prevents loss of light transmitted to the rear surface of the LCD panel 31.
The diffusion plate 17 scatters incident light to uniformly distribute the light onto the optical sheet 19. The optical sheet 19 collects the incident light from the diffusion plate 17 to maximize the brightness of the light and then transmits the light onto the LCD panel 31, thereby displaying an image on the LCD panel 31.
As mentioned above, in the backlight unit of the related art, the lower frame 11 is formed of a material having an excellent thermal conductivity. Therefore, a great amount of heat is transmitted to the panel supporting portion 11b of the lower frame 11. As apparent from the illustration of FIG. 1, heat transmitted to the lower frame 11 is partially transmitted to the upper frame 41 and the guide panel 21. Since the lower frame 11 is formed of a material having an excellent thermal conductivity, a great amount of heat is transmitted to the LCD panel 31. Accordingly, temperature is drastically increased in LCD panel 31.
In cases of a light source emitting a large amount of heat, such as a light emitting diode, the temperature of the LCD panel 31 can rise above the temperature at which the liquid crystal material begins to degrade. Additionally, reliability of the backlight is decreased due to the rising temperature as the heat affects consumption of power regardless of the optical source. Furthermore, severe heat changes of the LCD panel can generate wrinkles in the optical sheets very sensitive to thermal expansion, and thus cause inferior picture quality.